Systems and methods for direct particle sampling

ABSTRACT

Implementations described and claimed herein provide systems and methods for sampling particles from air. In one implementation, an inlet opening is defined in a proximal end of a cassette top, and the inlet opening has an inlet diameter. An internal surface extends along an airflow curve from the inlet opening to an internal cavity. A sampling substrate is formed by at least one grid attached to a filter. The sampling substrate is disposed in the internal cavity at an internal distance from the inlet opening. The inlet opening and the airflow curve of the internal surface generate an airflow of the air to the sampling substrate. The sampling substrate collects a set of the particles from the air, and the inlet diameter, the airflow, and the internal distance dictate a cutoff diameter of the set of particles collected from the air by the sampling substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 to U.S.Provisional Application No. 62/631,132, entitled “Sampler Design forNanoparticles and Respirable Particles with Direct Analysis Feature” andfiled on Feb. 15, 2018, which is specifically incorporated in itsentirety herein.

FIELD

Aspects of the present disclosure relate generally to the directcollection and analysis of airborne particles and more particularly tothe collection of particles in the nanometer and respirable sizesdirectly onto a filter and a grid, where the particles may be directlyanalyzed using a transmission electron microscope on the grid and ascanning electron microscope on the filter.

BACKGROUND

Nanomaterials may have toxicological effects, presenting a risk of harmto the environment and humans. Airborne nanomaterials can expose humansto such effects through inhalation. To assess the exposure, a particlesampler is typically used where size selective sampling separatesnanoparticles from larger particles for collection. Generally,nanoparticles may be collected through indirect or direct methods.Indirect methods are typically plagued by labor intensity, particletransfer, and the potential for particle agglomerate changes though thepreparation process. Direct methods involve directly depositingnanoparticles from air on a collection device, such as an electrostaticprecipitation sampler (ESP) or thermophoretic sample (TPS). ESP and TPSdevices typically only collect particles with diameters smaller than amicrometer in the nanometer size range, and only some of the particlesdrawn into the device are collected, with the rest directed out of thedevice with the exhaust air. As such, neither approach conventionallyprovides an accurate assessment of exposure of potentially toxicrespirable particles.

It is with these observations in mind, among others, that variousaspects of the present disclosure were conceived and developed.

SUMMARY

Implementations described and claimed herein address the foregoingproblems by providing systems and methods for sampling particles fromair. In one implementation, a cassette housing is formed from a cassettetop and a cassette bottom. An internal cavity is formed within thecassette housing. An inlet opening is defined in a proximal end of thecassette top, and the inlet opening has an inlet diameter. An internalsurface is defined in the top cassette. The internal surface extendsalong an airflow curve from the inlet opening to the internal cavity. Afilter has a plurality of pores, and a sampling substrate formed by atleast one grid attached to the filter. The sampling substrate isdisposed in the internal cavity at an internal distance from the inletopening. The inlet opening and the airflow curve of the internal surfacegenerate an airflow of the air to the sampling substrate. The samplingsubstrate collects a set of the particles from the air, and the inletdiameter, the airflow, and the internal distance dictate a cutoffdiameter of the set of particles collected from the air by the samplingsubstrate.

In another implementation, air is drawn into a cassette housing throughan inlet opening. The inlet opening has an inlet diameter. An airflow ofthe air to a sampling substrate is generated using the inlet opening andan internal surface extending along an airflow curve from the inletopening to the sampling substrate. The sampling substrate is formed byattaching at least one grid to a filter and is disposed at an internaldistance from the inlet opening. A cutoff diameter of the particles isdictated based on the inlet diameter, the airflow, and the internaldistance. A set of the particles is collected from the air on thesampling substrate based on the cutoff diameter.

In another implementation, a first image of a first set of collectedparticles captured using a transmission electron microscope on a grid isreceived. The first set of collected particles is collected directly onthe grid of a sampling substrate. A second image of a second set ofcollected particles captured using a scanning electron microscope on afilter is received. The second set of collected particles is collecteddirectly on the filter of the sampling substrate. The sampling substrateis disposed at an internal distance from an inlet opening defined in acassette housing. The inlet opening has an inlet diameter. A cutoffdiameter of the first set of collected particles and the second set ofcollected particles is dictated by the inlet diameter, the internaldistance, and an airflow generated using the inlet opening and aninternal surface extending along an airflow curve from the inlet openingto the sampling substrate. A particle count of the first set ofcollected particles and the second set of collected particles isdetermined by analyzing the first image and the second image using atleast one computing unit. One or more particle sizes of the first set ofcollected particles and the second set of collected particles isdetermined by analyzing the first image and the second image using theat least one computing unit. An airborne particle concentration of theair is determined based on the particle count and the one or moreparticle sizes.

Other implementations are also described and recited herein. Further,while multiple implementations are disclosed, still otherimplementations of the presently disclosed technology will becomeapparent to those skilled in the art from the following detaileddescription, which shows and describes illustrative implementations ofthe presently disclosed technology. As will be realized, the presentlydisclosed technology is capable of modifications in various aspects, allwithout departing from the spirit and scope of the presently disclosedtechnology. Accordingly, the drawings and detailed description are to beregarded as illustrative in nature and not limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side perspective view of an example particle sampler.

FIG. 2 is a side view of the particle sampler.

FIG. 3 shows an exploded side view of the particle sampler.

FIG. 4A shows a bottom perspective view of a cassette top of theparticle sampler.

FIG. 4B illustrates a top perspective view of a cassette bottom of theparticle sampler.

FIG. 5 depicts an example air sampling system.

FIG. 6 illustrates example operations for sampling particles from airusing an air sampler.

FIG. 7 illustrates example operations for analyzing particles collectedusing an air sampler.

FIG. 8 shows a computing system having one or more computing units thatmay implement aspects of the presently disclosed technology.

DETAILED DESCRIPTION

Aspects of the presently disclosed technology relate to systems andmethods for sampling particles from air. In one aspect, an air samplercollects particles in the nanometer and respirable sizes directly onto amembrane filter and a transmission electron microscope (TEM) gridthrough diffusion. The TEM grid is secured to the membrane filter, forexample, at a center to form a sampling substrate. The samplingsubstrate is disposed in an internal cavity of a cassette housing. Aninlet opening is defined in the cassette housing into which air isdrawn. The cassette housing has a unique geometry, including a diameterof the inlet opening and an internal surface extending from the inletopening to the internal cavity. The unique geometry of the cassettehousing generates a resulting air flow to the sampling substrate, whichtogether control a cutoff diameter of the collected particles. Thecutoff diameter may ensure that only respirable particles are sampled.The air sampler may be worn by an individual as a personal air samplerfor collecting particles in various environmental settings. Particlescollected by the air sampler are directly analyzable by a TEM on thegrid and by a scanning electron microscope (SEM) on the filter. Exposuremay be assessed through particle counts and elemental compositionanalysis of images captured by the TEM and/or SEM.

For a detailed description of an example air sampler 10, reference ismade to FIGS. 1-4B. In one implementation, the air sampler 10 includes acassette housing 100 having a proximal end 102 and a distal end 104. Theair sampler 10 may be manufactured through a variety of manufacturingprocesses. For example, the air sampler 10 may be manufactured by 3Dprinting through selective laser sintering (SLS) of nylon powder.Electrostatic effects of nonconductive nylon cyclone may have apotential to affect particle collection. However, nylon powder utilizedin 3D printing does not have this affect and may be altered forcollecting conductivity-sensitive substances. Other manufacturingmethods and systems are contemplated. The air sampler 10 is verylightweight with a low sampling air flow and thus is easily used as apersonal sampler in the field for personal exposure and/or environmentalsampling.

The cassette housing 100 is formed from a cassette top 106 and acassette bottom 108. The cassette top 106 and cassette bottom 108 may beone integral piece forming the cassette housing 100 or separate piecesconnected to each other, which may be releasably or permanently fixed toeach other. The cassette housing 100 forms an interior, which mayinclude an internal cavity 110. The internal cavity 110 may be formedthrough a connection between the cassette top 106 and the cassettebottom 108. For example, the cassette bottom 108 may include aprotrusion 112 receivable into the cassette top 106. The protrusion 112defines a distal portion of the internal cavity 110 that when theprotrusion 112 is received in the cassette top 106 forms the internalcavity 110.

In one implementation, the top cassette 106 includes an inlet 114disposed at the proximal end 102 of the cassette housing 100 and anoutlet 116 disposed at the distal end 104 of the cassette housing 100.The inlet 114 includes an inlet opening 118 through which air is drawninto an interior of the cassette housing 100 and directed through theinternal cavity 110. The outlet 116 includes an outlet opening 120through which air is expelled from the interior of the cassette housing100.

The internal geometry of the cassette housing 100 generates an air flowand controls a direction of the air flow from the inlet opening 118through the internal cavity 110 and out the outlet opening 120. In oneimplementation, in the interior of the cassette housing 100, thecassette top 106 includes an internal surface 122 extending along anairflow curve from the inlet opening 118 to the internal cavity 110 inwhich a sampling substrate 124 is disposed. The airflow curve includesone or more surface contours. For example, the airflow curve may besmooth and extend along a first contour outwardly from and generallytraverse to the inlet opening 118 and a second contour curving outwardlyand distally from the first contour to the internal cavity 110. Theinlet opening 118 and the airflow curve of the internal surface 122generate an airflow of air drawn into the interior of the cassettehousing through the inlet opening 118. The airflow is directed from theinlet opening 118 to the sampling substrate 124.

The inlet opening 118 has an inlet diameter 134, and the samplingsubstrate 124 is disposed an internal distance 136 from the inletopening 118. In one implementation, the inlet diameter 134, the airflow,and the internal distance 136 dictate a cutoff diameter of a set ofparticles collected from the air by the sampling substrate 124. Thecutoff diameter may be a mass median aerodynamic diameter (MMAD) ofapproximately 3.8 μm, which captures particles in the nanometer andrespirable sizes. The maximum aerodynamic diameter entering the airsampler 10 may be approximately 8 μm.

In one implementation, the sampling substrate 124 includes a filter 126and a grid 128. The grid 128 is directly attached to the filter 126using tape, an adhesive, a mechanical attachment, and/or otherattachment mechanisms. In one implementation, the grid 128 is attachedto a center of the filter 126 proximal to the inlet opening 118.However, the grid 128 may be attached at any location of the filter 126.Further, there may be a plurality of grids 128 disposed at variouslocations on the filter 126. The grid 128 may be a filmed TEM grid, andthe filter 126 may be a capillary pore membrane filter. As the airflowis directed across the sampling substrate 124, nanoparticles are drawnacross the grid 128, a first set of those particles migrate to thesurface of the grid 128 by diffusion. A second set of particlesincluding the remaining nanoparticles and larger particles below thecutoff diameter are collected on the filter 126 by diffusion. Stateddifferently, the grid 126 collects primary and small agglomerates ofsubmicrometer nanoparticles via Brownian motion as the airflow isdirected across a proximal end of the grid 128, and the filter 126collects particles over a wide size range via diffusion and sieving asthe airflow is directed through pores of the filter 126. Nanoparticlesand larger particles are collected on a proximal surface of the filter126 between the pores.

The airflow is directed through the sampling substrate 124 in theinternal cavity 110 along an outlet curve 132 to the outlet opening 120,which expels the air. In one implementation, the outlet curve 132extends distally along one or more contours from the internal cavity 110to the outlet opening 120. The internal cavity 110 extends between aproximal surface 152 and a distal surface 148. The internal cavity 110is defined in the protrusion 112, which extends between the proximalsurface 152 and a surface of a shelf 130 of the cassette bottom 108. Theshelf 130 extends radially outwardly from the protrusion 112 and isadapted to receive a distal surface 150 of the cassette top 106. Whenthe shelf 130 meets the distal surface 150, the protrusion 112 extendsproximally into a proximal cavity 140, and the proximal surface 152 ofthe cassette bottom 108 meets a proximal internal surface 138 of thecassette top 106.

The proximal cavity 140 and/or the internal cavity 110 may house one ormore components to support the sampling substrate 124 or otherwisefacilitate the collection of particles by the sampling substrate 124.For example, the sampling substrate 124 is supported by a support screen144, which may be metal, and disposed between a proximal O-ring 142 anda distal O-ring 146.

As described herein, the air sampler 10 collects particles in thenanometer and respirable size ranges with the aerodynamic cutoffdiameter of approximately 3.8 μm for respirable particles. The particlesize entering the cassette housing 100 is determined based on the inletdiameter 134 of the inlet opening 118 and the airflow. A shape of theinlet opening 118 may further impact the particle size entering thecassette housing 100, thereby providing pre-separation of the particles.For example, the inlet opening 118 may be round. Such a round shape maygenerate an aspiration efficiency following a sigmoidal curve like arespirable pre-separator curve with the cutoff diameter being dictatedby the inlet diameter 134, the airflow, and/or the internal distance136. The geometry of the air sampler 10 is such that inlet aspirationefficiency is minimally affected by sampling direction.

In one particular implementation, the inlet opening 118 is circular inshape, with the inlet diameter is approximately 2 mm, and the internaldistance 136 is approximately 6 mm between the sampling substrate 124and the inlet opening 118 at a proximal end of the cassette housing 100,when the cassette housing 100 is approximately 25 mm in diameter. Theinternal surface 122 is smooth, thereby generating a smooth airstreamline of the airflow for particles moving from the inlet opening118 towards the sampling substrate 124. As described herein, each ofthese factors dictates the sizes of particles entering the air sampler10 and the cutoff diameter of particles collected on the samplingsubstrate 124. In one particular implementation, the filter 126 of thesampling substrate 124 has a diameter of approximately 25 mm with poreshaving a size of approximately 0.22 μm and made from polycarbonate. Thegrid 128 is a TEM-copper-grid in 400 mesh with a SiO₂ coated film. Itwill be appreciated that other coated films and materials for the grid128 may be used. The sampling substrate 124 may form a 23 mm diametercollecting area.

As described herein, particles are collected on both the filter 126 andthe grid 128 through diffusion. In one implementation, airflow isdirected through the sampling substrate 124 and aerosol particles aredeposited on the grid 128 via Brownian diffusion. The diffusion of aparticle is a function of the diffusion coefficient D, which may begiven by the Stokes-Einstein equation:

${D = \frac{{kTC}_{c}}{3\pi \; \eta \; d_{p}}},$

with k being Boltzman's constant, T being the absolute temperature,C_(c) being the Cunningham slip correction factor, η being the gasviscosity, and d_(p) being the particle diameter.

It will be appreciated that at standard conditions, including a constanttemperature and viscosity, the diffusion coefficient depends only on thediameter, such that diffusion increases as diameter decreased. SinceBrownian motion is random, a collection of aerosol particles of the samediameter will be displaced different distances along an arbitrary axisin any given time interval. The distance particles travel under Brownianmotion approximates a normal distribution. Thus, the root mean square(rms) average distance for movement of a collection of particles in timet by definition is equal to the standard deviation (o) of thedistribution and is given by:

σ=X _(rms)=√{square root over (2Dt)}  (2)

According to these relationships, during the time t that a particlespends passing the grid 128, smaller particles can expect to travel alarger average distance than larger ones, so a great portion of smallerparticles would be expected to be found on the grid 128. Thus,nanoparticles, due to their large Brownian displacement, have a highprobability of striking a TEM grid element as they pass by the grid 128.

In one particular non-limiting example, using an airflow ofapproximately 0.3 L/min through the cassette housing 100, the airflow isdirected towards the grid 128 at a velocity of approximately 1 cm/s. Airstreamlines striking a center of the grid 128 will be directed towardedges of the grid 128. If the grid 128 has a radius of 1.5 mm, airstriking the center of the grid 128 will take approximately 0.15 s toreach the edge. Based on the relationships above, a 10 nm diameterparticle, for example, has an rms displacement (X_(rms)) of 0.13 mm in0.15 s. Since the X_(rms) is also by definition its standard deviation,and 32% of any distribution values are greater than the standarddeviation, 32% of the particles will have moved farther than thisdistance while passing over the grid 128. Half of these particles willmove away from the surface of the grid 128 and half of them (16%) willmove toward the surface of the grid 128 and deposit on the filmedopening space of the grid 128. Thus, 10 nm particles travelling onstreamlines that pass within a few tenths of a millimeter of the surfaceof the grid 128 will have a high probability of being collected. Inaddition, flow so close to the surface of the grid 128 is likely to bein the frictional boundary layer, meaning that the velocities are lowerthan the free-stream velocities, the residence times are longer, andmore collection by diffusion will occur. The calculated rmsdisplacements range from 0.32 mm for a 4 nm diameter particle to 0.01 mmfor a 300 nm diameter particle as shown in the table below, withparticle diameters and corresponding parameters including diffusioncoefficient (D), coefficient of diffusion collection (N_(D)), efficiencyof diffusion (E_(D)), and root mean square displacement (X_(rms)):

d (μm) D (m²/s) N_(D) E_(D) X_(rms) (mm) 0.004 3.35E−07 9377.3 1.00 0.320.005 2.15E−07 6018.3 1.00 0.25 0.006  1.5E−07 4198.8 1.00 0.21 0.0088.46E−08 2368.1 1.00 0.16 0.01 5.45E−08 1525.6 1.00 0.13 0.015 2.45E−08685.8 1.00 0.09 0.02  1.4E−08 391.9 1.00 0.06 0.03 6.39E−09 178.9 1.000.04 0.04 3.69E−09 103.3 1.00 0.03 0.05 2.43E−09 68.0 1.00 0.03 0.061.73E−09 48.4 1.00 0.02 0.08 1.03E−09 28.8 1.00 0.02 0.1 6.94E−10 19.41.00 0.01 0.15 3.51E−10 9.8 1.00 0.01 0.2 2.23E−10 6.2 1.00 0.01 0.31.23E−10 3.4 1.00 0.01 0.4 8.31E−11 2.3 1.00 0.00 0.5 6.24E−11 1.7 1.000.00 0.6 4.98E−11 1.4 0.99 0.00 0.8 3.54E−11 1.0 0.98 0.00 1 2.74E−110.8 0.95 0.00

As illustrated with these values, particles across the entire nanometersize range will have a measurable displacement while passing over thesurface of the grid 128, and particles throughout this size range willdeposit on the grid 128 due to diffusion. These values indicate that asignificant number of the nanometer-sized particles in the air flowimpinging on the grid 128 will move to the grid surface and becollected.

With respect to the filter 126, in one implementation, particles enterthe cassette housing 100 through the inlet opening 118 are distributedonto the filter 126, with particle agglomerates larger than the poresize (˜220 nm) of the filter 126 being deposited by sieving. Particlessmaller than the pore size will be deposited onto the surface of thefilter 126 between pores by Brownian motion. The filter 126 may be aNuclepore polycarbonate filter or otherwise a membrane filter with theefficiency of diffusion, E_(D), on the filter 126 being:

E_(D)=1−0.081904 exp (−3.6568N_(D))−0.09752 exp (−22.3045N_(D))−0.03248exp (−56.95 N_(D))−0.0157 exp (−107.6N_(D))− . . . , where thecoefficient of diffusive collection, N_(D), is greater than 0.01.

The remaining terms will approach zero, with the coefficient ofdiffusive collection being given by:

${N_{D} = \frac{LDP}{R_{0}^{2}q}},$

where L is we niter thickness (m), D is the diffusion coefficient, P isthe filter porosity, R₀ is the pore radius of a clean filter (m), and qis the face velocity of air at the filter 126 (m/s).

In one particular non-limiting example, the filter 126 is a 0.22 μmporous polycarbonate filter with a thickness of 25 μm (Sigma), aporosity of 13.8%, a pore radius of 0.11 μm, and q of 0.01 m/s. Thediffusion efficiency of the filter 126 is 100% for particles withdiameters of 500 nm (0.5 μm) or smaller, and greater than 95% forparticles of 1 μm to 0.5 μm as shown in the table above. Particles withdiameters greater than the pore size (220 nm) will be collected with100% efficiency.

Motion of a particle entering the inlet opening 118 and inside thecassette housing 100, can be characterized using the Stokes number(Stk). In this equation, τ is the relaxation time (s), U₀ is the airvelocity, and d_(c) is the cylinder diameter of airflow which has beenanalyzed separately for both the inlet diameter 134 (e.g., 2 mm) anddiameter of the cassette housing 100 (e.g., 23 mm).

${Stk} = {\tau \frac{U_{0}}{d_{c}}}$

${\tau = \frac{\rho_{p}d_{p}^{2}C_{C}}{18\; \eta}},$

where ρ_(p) is particle density (kg/m³).

When Stk>>1, particles continue moving in a straight line when the airturns, and when Stk<<1, particles follow the air streamlines. Theparticle motion behavior at the inlet opening 118 determines theparticle sizes entering the inlet 114, as they follow and deviate fromthe air streamlines. This determines the maximum size of particles to besampled by the air sampler 10, or in other words, the largest particlethat can follow the sharply-curving streamlines at the inlet opening 118and the internal surface 122.

Turning to FIG. 5, in one implementation, an air sampling system 200includes the air sampler 10 connected to a sampling pump 202 with anozzle 204. The air sampling system 200 may be worn by an individualand/or used to analyze air and particle exposure in a variety ofenvironmental settings, such as mines, labs, plants, and/or the like.Examples of emitted particles in various settings that may be sampledinclude, laser printer printing, handling fumed silica powder,disturbing bulk samples of carbon nanotubes, and particles released frommining activity in an experimental metal mine, among many otherexamples. Further, the sampled particles may be fibrous and/ornon-fibrous particles, and spherical primary particles such as metal,nonmetal or metal-oxide nanoparticles, primary fibrous particles, andagglomerates of all these particles may be collected using the airsampler 10. The sampling pump 202 generates a pressure that draws airfrom the environment through the inlet 114 into an interior of thecassette housing 100 where particles are collected. As described herein,the sampling pump 202 may be running at approximately 0.3 L/min airflow. The air is expelled from the cassette housing 100 into the nozzle204.

The air sampling system 200 may be used to sample a variety of differenttypes of particles. In one particular non-limiting example forillustration purposes only, the air sampling system 200 may be used tosample Crystalline aluminum oxide (Al₂O₃) nanopowder with a density of3600 kg/m³ and primary particle size of 40 nm. Such a nanopowdergenerally includes a wide range of aerosol particle sizes fromindividual primary particles to small and large agglomerates. The powdermay become aerosolized through agitation and directed with the air,moving at the air velocity, which may be for example, 1 to 7.6 cm/s (2to 15 ft/min) for horizontal direction and from 0 to 3 cm/s (0 to 6ft/min) for vertical direction. In this example, aerosol generationoccurs for approximately 40 minutes. The particles are collected on thefilter 126 and the gird 128 through diffusion, as described herein.After sampling, the sampling substrate 124 is removed from the cassettehousing 100 for direct analysis using a TEM and/or SEM.

A variety of imaging protocols may be used to directly capture images ofthe particles on the filter 126 and/or the grid 128 using the TEM and/orSEM, and a variety of imaging software executed by at least onecomputing device, such as those described with respect to FIG. 8, may beused to analyze the images.

In one implementation, TEM imaging protocol may be utilized to analyzegrid 128, including capturing five low magnification (50×) images toencompass the entire grid 128 (e.g., center, and four quadrants). Fourgrid spaces may be selected equidistant two spaces from the grid centerand imaged at 500×. These chosen grid spaces may be imaged by traversing(9-12 images per grid) the grid space at 5000 x until 300 particles areimaged. For analysis of the filter 126 using the SEM, the filters wereprepared by cutting a ⅛^(th) slice of the filter 126 and coating with 10nm of gold. Three sections of the filters were imaged (e.g., center,middle, and edge) at 9000-25000× until 300 particles were imaged persample. Particle images were analyzed using imaging software to countand measure particle size. The particle sizes were organized following13 bin ranges within 10 nm to 420 nm and 14 bin ranges above 420 nm to10 μm, to plot particle count frequency with size distribution andcumulative percent particle counts.

More particularly, imaging protocol to analyze the grid 128 using TEMincludes removing the grid 128 from the filter 126 and placing the grid128 on a TEM grid holder. The holder is then placed inside the TEM. Insome cases, the grid 128 can be stored in a grid storage case beforeperforming the analysis. A plurality of low magnification images (e.g.,5 images at a magnification of 50×) are captured using the TEM toencompass the entire grid 128 (e.g., center, and four quadrants). Aplurality of grid spaces (e.g., 4 grid spaces) disposed equidistant(e.g., two spaces) from the grid center are selected and imaged at anincreased (e.g., 500×) magnification. For size-fractioned particlecounting analysis, approximately 300 or more particles are included witha first range of magnification and a second range of magnification. Forparticles with a typical diameter smaller than 1 μm, the chosen gridspaces are imaged by traversing the grid space at the firstmagnification range (e.g., between 5000× and 50,000×) untilapproximately 300 or more particles/agglomerates are imaged. Forparticles with a typical diameter that is larger than 1 μm, the chosengrid spaces are imaged by traversing the grid space at the secondmagnification range (e.g., between 500× and 6000×) until approximately300 or more particles/agglomerates are imaged.

The imaging protocol to analyze the filter 126 using SEM includespreparing the filter 126 by cutting a slice (e.g., ⅛^(th)) of the filter126. The slice may be cut for example in a triangle piece across fromthe center to the edge of the filter 126. However, other shapes arecontemplated. The slice is coated with a coating material, such as 10 nmof gold. A plurality of sections (e.g., center, middle, and edge) of theslice of the filter 126 is imaged at a magnification of 2000× to 25000×.Images are captured until approximately 300 or moreparticle/agglomerates are included in images per sample.

The images captured from the TEM and/or SEM are analyzed using imagingsoftware to count and measure particle area for diameter conversion.Gatan digital micrograph may be used for counting and measuring particlediameters for size bin sorting. In one implementation, particle countsare organized by following the bin ranges of a real time instrument(RTI) to count the number of particles in each bin range. In thisimplementation, particle diameter is estimated as the equivalentdiameter of measured two-dimensional area of particle image. In anotherimplementation, particle size is organized using their maximum crosswisedimension and placed into size bins of smaller than 1 μm, 1 to 2 μm, 2to 5 μm and 5 to 10 μm. For analyzing fibers, individual fibers with anaspect ratio larger than 3 to 1 may be counted. The clusters of fibrousparticle are measured using their maximum crosswise dimension and thenplace into size bins of smaller than 1 μm, 1 to 2 μm, 2 to 5 μm, 5 to 10μm, and larger than 10 μm.

In one implementation, based on an assumption of approximately 100%collection of particles at the center of the filter, an airborneparticle number concentration entering the air sampler 10 can bedetermined. For the air sampler 10, an effective filter area of 415 mm²(23 mm diameter) is used and an opening area of 1.37×10⁻³ mm² for thegrid 128 is used. The volume is equal to flowrate (e.g., 0.3 L/min)multiply by sampling time (min). The overall particle count in the rangeof 10 nm to 10 μm or the particle count in the specific size bin can beconverted using the following equation to obtain overall particle numberconcentration. A size-fractioned particle number concentration iscalculated use the number of particle of a specific size bin.

$\frac{{Total}\mspace{14mu} {Number}\mspace{14mu} {of}\mspace{20mu} \frac{Particles}{Fiber}\mspace{14mu} {Structures}}{{cm}^{3}} = {\frac{\begin{matrix}{{Total}\mspace{14mu} {Number}\mspace{14mu} {of}\mspace{14mu} {Structures}} \\{{from}\mspace{14mu} 4\mspace{14mu} {Grid}\mspace{14mu} {Openings}}\end{matrix}}{{Number}\mspace{14mu} {of}\mspace{14mu} {Grid}\mspace{14mu} {Opening}\mspace{14mu} {Counted}\mspace{14mu} (4)} \times \frac{1}{{Volume}(L)} \times \frac{{Effective}\mspace{14mu} {Filter}\mspace{14mu} {Area}\mspace{14mu} \left( {mm}^{2} \right)}{{One}\mspace{14mu} {Grid}\mspace{14mu} {Opening}\mspace{14mu} {Area}\mspace{14mu} \left( {mm}^{2} \right)} \times \frac{1L}{1000\mspace{14mu} {cm}^{3}}}$

In the illustrative example of collection of aluminum oxide particles,the particles had an average primary size of 40 nm diameter with smallprimary particles less than 40 nm and agglomerates ranged in nanometerto micrometer sizes. The concentration of each size bin and standarddeviation data are included in the table below:

Particle Particle Diameter SMPS Conc. SMPS Diameter OPS Conc. OPS (nm)[Particle/cm³] STD (nm) [Particle/cm³] STD 12 12.2 19.7 337 40.1 83.5 1520.5 42.0 419 6.3 13.1 21 33.3 70.8 522 3.1 6.5 27 57.5 72.3 650 1.1 2.237 79.2 112.0 809 0.9 2.0 49 94.8 329.8 1007 1.2 2.7 65 132.7 479.1 12540.7 1.5 87 263.0 1143.3 1562 1.2 2.7 116 450.8 1763.7 1944 2.1 4.7 154572.6 1717.7 2421 2.9 6.7 205 450.4 1022.6 3014 3.9 9.0 274 243.2 489.63752 5.6 12.9 365 114.8 273.4 4672 7.3 16.9 5816 7.8 18.3 7242 8.7 19.89016 9.9 22.6

The size distributions of generated airborne aluminum oxide particlesmeasured by a Nanoscan Scanning Mobility Particle Sizer (SMPS) are shownin the size range from 10 to 420 nm with the mode size measured to be154 nm. The Optical Particle Sizer (OPS) measurements showed increasingconcentrations toward to both minimum and maximum sizes measured by theinstrument. The average particle number concentration during 62 min (62data) of sampling measured by the Nanoscan SMPS (10-420 nm) and by OPS(0.3-10 μm) were 237 particles/cm³ and 7 particles/cm³, respectively.The peak emission showed concentrations in a range from about 50 togreater than 4000 particles/cm³ for particles smaller than 420 nm, andthe particle counts for larger particles above 0.3 μm ranged from a fewparticles to about 80 particles/cm³. The particle counts were aluminumoxide particles only with essentially 0 particles from the ambientbackground. These emissions represented a low concentration rangerepresentative of a typical exposure in none occupational environment.

Collected particles on the filter and grid may be analyzed separatelyusing SEM and TEM. Small particles and agglomerates similar to orsmaller than the pore size of the filter 126 are collected between poresand/or trapped on pores. The grid 128 collects particles on the gridspace, with many particles collected on each grid space, most of whichbeing primary particles or small agglomerates. A plurality of images ofthe collected particles on the filter 126 and the spaces of the grid 128are taken to obtain a minimum of 300 particles being counted andanalyzed to obtain their sizes. Particles on the filter 126 range insize from 36 to 5,800 nm, and particles on the grid 128 range in sizefrom 15 to 1,560 nm. The particle mean diameters are 793 nm and 130 nm,respectively, for aluminum oxide particles on the filter 126 and thegrid 128. Based on the particle count size distribution, particlecharacteristics are analyzed. For the example of the aluminum oxideparticles, the characteristics are summarized below:

Al₂O₃ particles Particles on filter Particles on grid Particle meandiameter 793 nm 130 nm Mean diameter standard deviation 591 nm 127 nmGeometric mean diameter, d_(g) 641 nm 92 nm Geometric standarddeviation, σ_(g) 1.97 2.30 Count median diameter, CMD 500 nm 100 nm Massmedian diameter, MMD 1,987 nm 793 nm Mass median aerodynamic 3,770 nm1,505 nm diameter, MMAD

The geometric mean diameter, d_(g), and the geometric standarddeviation, σ_(g), may be calculated as follows:

${\ln \mspace{14mu} d_{g}} = \frac{\Sigma \; n_{i}\ln \; d_{i}}{N}$${\ln \mspace{14mu} \sigma_{g}} = \left( \frac{\Sigma \; {n_{i}\left( {{\ln \; d_{i}} - {\ln \; d_{g}}} \right)}^{2}}{N - 1} \right)^{1/2}$

CMD may be obtained from a cumulative percentile at 50% cumulativeparticle size and converted to MMD of aluminum oxide particles utilizingthe Hatch-Choate conversion equation:

MMD=CMD exp(3 ln² σ_(g))

The mass median aerodynamic diameter (MMAD) may then be determined asfollows:

d_(a)=d_(p) (ρ_(p)/ρ₀)^(1/2), where d_(a) is aerodynamic diameter, d_(p)is particle diameter, ρ_(p) is particle density, and ρ₀ is standardparticle density which is 1 g/cm³.

For aluminum oxide particles collected on the polycarbonate filter ofthis sampler, the CMD was 500 nm (0.5 μm) and the MMD was approximately2 μm. For a lognormal count distribution, d_(g) is equal to the CMD. Ascan be understood from the summarized characteristics, d_(g) and CMDdiffer because the underlying distribution is not actually log-normal.The equivalent MMAD of aluminum oxide particles would be approximately3.8 μm.

When compared to an RTI, such as a Nanoscan SMPS or OPS, the air sampler10 had smaller particle modes at 65 nm and 116 nm. The SMPS, forexample, measures such particles at 154 nm. Moreover, the size range ofparticles from image analysis for the air sampler 10 extended above 420nm, the maximum size measured by SMPS. Additionally, the SMPS cannotdirectly compare the absolute sizes of measured particles. The SMPSmeasures the electrical mobility size of particles, which is correlatedwith the electrical charge on the surface of three dimensionalparticles, to determine particle diameter. However, the size analysis ofcollected particles is based on two-dimensional particle images used tomeasure the area of imaged individual particles and agglomerates, whichis then used to calculate the diameter of equivalent sphericalparticles. The OPS shows increased particle concentration toward to themaximum size of 10 μm. On the other hand, the air sampler 10 collectsparticle sizes up to 8 μm aerodynamic diameter such that that largerparticle agglomerates do not enter the inlet opening 118. These factorscreate differences in measurements by these devices.

Motion of a particle while entering the cassette housing 100 through theinlet opening 118 and inside the cassette housing 100 will vary by itsdiameter. This relationship can be interpreted using the particle Stokesnumber, as described herein with example data included in the tablebelow:

Al₂O₃ Particles Standard Density Particles Stk at Stk in Stk at Stk in d(μm) τ (sec) inlet cassette τ (sec) inlet cassette 0.005 2.76E−101.40E−06 8.00E−11 7.67E−11 3.89E−07 2.22E−11 0.010 1.10E−09 5.60E−063.20E−10 3.07E−10 1.55E−06 8.89E−11 0.015 2.49E−09 1.26E−05 7.20E−106.91E−10 3.50E−06 2.00E−10 0.020 4.42E−09 2.24E−05 1.28E−09 1.23E−096.22E−06 3.56E−10 0.027 8.06E−09 4.08E−05 2.33E−09 2.24E−09 1.13E−056.48E−10 0.036 1.43E−08 7.25E−05 4.15E−09 3.98E−09 2.02E−05 1.15E−090.050 2.76E−08 1.40E−04 8.00E−09 7.67E−09 3.89E−05 2.22E−09 0.0654.67E−08 2.37E−04 1.35E−08 1.30E−08 6.57E−05 3.76E−09 0.087 8.36E−084.24E−04 2.42E−08 2.32E−08 1.18E−04 6.73E−09 0.115 1.46E−07 7.40E−044.23E−08 4.06E−08 2.06E−04 1.18E−08 0.154 2.62E−07 1.33E−03 7.59E−087.28E−08 3.69E−04 2.11E−08 0.205 4.64E−07 2.35E−03 1.35E−07 1.29E−076.53E−04 3.74E−08 0.174 3.35E−07 1.69E−03 9.69E−08 9.29E−08 4.71E−042.69E−08 0.337 1.25E−06 6.36E−03 3.63E−07 3.49E−07 1.77E−03 1.01E−070.419 1.94E−06 9.83E−03 5.62E−07 5.39E−07 2.73E−03 1.56E−07 0.5223.01E−06 1.53E−02 8.72E−07 8.36E−07 4.24E−03 2.42E−07 0.650 4.67E−062.37E−02 1.35E−06 1.30E−06 6.57E−03 3.76E−07 0.809 7.23E−06 3.66E−022.09E−06 2.01E−06 1.02E−02 5.82E−07 1.007 1.12E−05 5.68E−02 3.25E−063.11E−06 1.58E−02 9.02E−07 1.254 1.74E−05 8.80E−02 5.03E−06 4.83E−062.45E−02 1.40E−06 1.562 2.70E−05 1.37E−01 7.81E−06 7.49E−06 3.79E−022.17E−06 1.944 4.18E−05 2.12E−01 1.21E−05 1.16E−05 5.88E−02 3.36E−062.421 6.48E−05 3.28E−01 1.88E−05 1.80E−05 9.11E−02 5.21E−06 3.0141.00E−04 5.09E−01 2.91E−05 2.79E−05 1.41E−01 8.08E−06 3.752 1.56E−047.88E−01 4.51E−05 4.32E−05 2.19E−01 1.25E−05 3.800 1.60E−04 8.08E−014.62E−05 4.43E−05 2.25E−01 1.28E−05 3.900 1.68E−04 8.51E−01 4.87E−054.67E−05 2.37E−01 1.35E−05 4.000 1.77E−04 8.96E−01 5.12E−05 4.91E−052.49E−01 1.42E−05 4.100 1.86E−04 9.41E−01 5.38E−05 5.16E−05 2.61E−011.49E−05 4.200 1.95E−04 9.87E−01 5.65E−05 5.41E−05 2.74E−01 1.57E−054.300 2.04E−04 1.04E+00 5.92E−05 5.68E−05 2.88E−01 1.64E−05 4.4002.14E−04 1.08E+00 6.20E−05 5.94E−05 3.01E−01 1.72E−05 4.500 2.24E−041.13E+00 6.48E−05 6.22E−05 3.15E−01 1.80E−05 4.672 2.41E−04 1.22E+006.99E−05 6.70E−05 3.39E−01 1.94E−05 5.000 2.76E−04 1.40E+00 8.00E−057.67E−05 3.89E−01 2.22E−05 5.500 3.34E−04 1.69E+00 9.68E−05 9.28E−054.70E−01 2.69E−05 6.000 3.98E−04 2.02E+00 1.15E−04 1.10E−04 5.60E−013.20E−05 6.500 4.67E−04 2.37E+00 1.35E−04 1.30E−04 6.57E−01 3.76E−057.000 5.41E−04 2.74E+00 1.57E−04 1.50E−04 7.62E−01 4.36E−05 7.5006.22E−04 3.15E+00 1.80E−04 1.73E−04 8.75E−01 5.00E−05 8.000 7.07E−043.58E+00 2.05E−04 1.96E−04 9.95E−01 5.69E−05 8.500 7.98E−04 4.04E+002.31E−04 2.22E−04 1.12E+00 6.42E−05 9.000 8.95E−04 4.53E+00 2.59E−042.49E−04 1.26E+00 7.20E−05 9.500 9.97E−04 5.05E+00 2.89E−04 2.77E−041.40E+00 8.02E−05 10.000 1.10E−03 5.60E+00 3.20E−04 3.07E−04 1.55E+008.89E−05

When particles enter the inlet opening 118 in the example above,aluminum oxide particles approximately 4 μm or smaller will follow theair streamline since the Stokes number is less than 1. Forstandard-density particles, diameters approximately 8 μm or smaller willfollow the air streamline in the inlet opening 118, since their Stokesnumber is less than 1. This determines the maximum particle sizeentering the cassette housing 100. Once particles enter into thecassette housing 100, all Stokes numbers are extremely small forparticles 10 μm or smaller. Thus, all particles will follow airstreamlines.

Contrary to other nanoparticle samplers, quantitative analysis can beperformed using the air sampler 10 since 100% of the particles enteringthe cassette housing 100 are collected on the sampling substrate 124.The particle number concentration can be estimated from the number ofparticles counted per image, the area of the grid opening that wasimaged, the sampling air flow, and the sampling time period to provide aquantitative analysis of size-fractioned particle concentration.

Referring to FIG. 6, example operations 300 for sampling particles fromair are shown. In one implementation, an operation 302 draws an into acassette housing through an inlet opening. The inlet opening has aninlet diameter. The operation 302 may draw the air into the cassettehousing using a personal sampling pump, for example. An operation 304generates an airflow of the air to a sampling substrate using the inletopening and an internal surface. The internal surface extends along anairflow curve from the inlet opening to the sampling substrate. Thesampling substrate is formed by attached a grid to a filter. The gridmay be attached to a center of a surface of the filter that is proximalto the inlet opening. The grid may be attached to the filter using tape.The sampling substrate is disposed at an internal distance from theinlet opening.

An operation 306 dictates a cutoff diameter of particles based on theinlet diameter, the airflow, and the internal distance. An operation 308collects a set of the particles from the air on the sampling substratebased on the cutoff diameter. The operation 308 may collect the set ofparticles through diffusion. The cutoff diameter may be a mass medianaerodynamic diameter of 3.8 μm.

Turning to FIG. 7, example operations 400 for sampling particles fromair are shown. In one implementation, an operation 402 receives a firstimage of a first set of collected particles captured using a TEM on agrid. The first set of collected particles may be collected directly onthe grid of a sampling substrate.

An operation 404 receives a second image of a second set of collectedparticles captured using an SEM on a filter. The second set of collectedparticles are collected directly on the filter of the samplingsubstrate. The sampling substrate is disposed at an internal distancefrom an inlet opening defined in a cassette housing. A cutoff diameterof the first and second sets of collected particles is dictated by aninlet diameter of the inlet opening, the internal distance, and anairflow generated using the inlet opening and an internal surfaceextending along an airflow curve from the inlet opening to the samplingsubstrate.

An operation 406 determines a particle count of the first and secondsets of collected particles by analyzing the first image and the secondimage using at least one computing unit. An operation 408 determines oneor more particle sizes of the first and second sets of collectedparticles by analyzing the first image and the second image using the atleast one computing unit. An operation 410 determines an airborneparticle concentration of the air based on the particle count and theone or more particles sizes.

Referring to FIG. 8, a detailed description of an example computingsystem 500 having one or more computing units that may implement varioussystems and methods discussed herein, such as the particle imageanalysis, is provided. The computing system 500 may be applicable tovarious computing or network devices to analyze particles captured inone or more images from a TEM, SEM, and/or other imaging devices. Itwill be appreciated that specific implementations of these devices maybe of differing possible specific computing architectures not all ofwhich are specifically discussed herein but will be understood by thoseof ordinary skill in the art.

The computer system 500 may be a computing system is capable ofexecuting a computer program product to execute a computer process. Dataand program files may be input to the computer system 500, which readsthe files and executes the programs therein. Some of the elements of thecomputer system 500 are shown in FIG. 8, including one or more hardwareprocessors 502, one or more data storage devices 504, one or more memorydevices 508, and/or one or more ports 508-510. Additionally, otherelements that will be recognized by those skilled in the art may beincluded in the computing system 500 but are not explicitly depicted inFIG. 8 or discussed further herein. Various elements of the computersystem 500 may communicate with one another by way of one or morecommunication buses, point-to-point communication paths, or othercommunication means not explicitly depicted in FIG. 8.

The processor 502 may include, for example, a central processing unit(CPU), a microprocessor, a microcontroller, a digital signal processor(DSP), and/or one or more internal levels of cache. There may be one ormore processors 502, such that the processor 502 comprises a singlecentral-processing unit, or a plurality of processing units capable ofexecuting instructions and performing operations in parallel with eachother, commonly referred to as a parallel processing environment.

The computer system 500 may be a conventional computer, a distributedcomputer, or any other type of computer, such as one or more externalcomputers made available via a cloud computing architecture. Thepresently described technology is optionally implemented in softwarestored on the data stored device(s) 504, stored on the memory device(s)506, and/or communicated via one or more of the ports 508-510, therebytransforming the computer system 500 in FIG. 8 to a special purposemachine for implementing the operations described herein. Examples ofthe computer system 500 include personal computers, terminals,workstations, mobile phones, tablets, laptops, personal computers,multimedia consoles, gaming consoles, set top boxes, and the like.

The one or more data storage devices 504 may include any non-volatiledata storage device capable of storing data generated or employed withinthe computing system 500, such as computer executable instructions forperforming a computer process, which may include instructions of bothapplication programs and an operating system (OS) that manages thevarious components of the computing system 500. The data storage devices504 may include, without limitation, magnetic disk drives, optical diskdrives, solid state drives (SSDs), flash drives, and the like. The datastorage devices 504 may include removable data storage media,non-removable data storage media, and/or external storage devices madeavailable via a wired or wireless network architecture with suchcomputer program products, including one or more database managementproducts, web server products, application server products, and/or otheradditional software components. Examples of removable data storage mediainclude Compact Disc Read-Only Memory (CD-ROM), Digital Versatile DiscRead-Only Memory (DVD-ROM), magneto-optical disks, flash drives, and thelike. Examples of non-removable data storage media include internalmagnetic hard disks, SSDs, and the like. The one or more memory devices506 may include volatile memory (e.g., dynamic random access memory(DRAM), static random access memory (SRAM), etc.) and/or non-volatilememory (e.g., read-only memory (ROM), flash memory, etc.).

Computer program products containing mechanisms to effectuate thesystems and methods in accordance with the presently describedtechnology may reside in the data storage devices 504 and/or the memorydevices 506, which may be referred to as machine-readable media. It willbe appreciated that machine-readable media may include any tangiblenon-transitory medium that is capable of storing or encodinginstructions to perform any one or more of the operations of the presentdisclosure for execution by a machine or that is capable of storing orencoding data structures and/or modules utilized by or associated withsuch instructions. Machine-readable media may include a single medium ormultiple media (e.g., a centralized or distributed database, and/orassociated caches and servers) that store the one or more executableinstructions or data structures.

In some implementations, the computer system 500 includes one or moreports, such as an input/output (I/O) port 508 and a communication port510, for communicating with other computing, network, or vehicledevices. It will be appreciated that the ports 508-510 may be combinedor separate and that more or fewer ports may be included in the computersystem 500.

The I/O port 508 may be connected to an I/O device, or other device, bywhich information is input to or output from the computing system 500.Such I/O devices may include, without limitation, one or more inputdevices, output devices, and/or environment transducer devices.

In one implementation, the input devices convert a human-generatedsignal, such as, human voice, physical movement, physical touch orpressure, and/or the like, into electrical signals as input data intothe computing system 500 via the I/O port 508. Similarly, the outputdevices may convert electrical signals received from computing system500 via the I/O port 508 into signals that may be sensed as output by ahuman, such as sound, light, and/or touch. The input device may be analphanumeric input device, including alphanumeric and other keys forcommunicating information and/or command selections to the processor 502via the I/O port 508. The input device may be another type of user inputdevice including, but not limited to: direction and selection controldevices, such as a mouse, a trackball, cursor direction keys, ajoystick, and/or a wheel; one or more sensors, such as a camera, amicrophone, a positional sensor, an orientation sensor, a gravitationalsensor, an inertial sensor, and/or an accelerometer; and/or atouch-sensitive display screen (“touchscreen”). The output devices mayinclude, without limitation, a display, a touchscreen, a speaker, atactile and/or haptic output device, and/or the like. In someimplementations, the input device and the output device may be the samedevice, for example, in the case of a touchscreen.

The environment transducer devices convert one form of energy or signalinto another for input into or output from the computing system 500 viathe I/O port 508. For example, an electrical signal generated within thecomputing system 500 may be converted to another type of signal, and/orvice-versa. In one implementation, the environment transducer devicessense characteristics or aspects of an environment local to or remotefrom the computing device 500, such as, light, sound, temperature,pressure, magnetic field, electric field, chemical properties, physicalmovement, orientation, acceleration, gravity, and/or the like. Further,the environment transducer devices may generate signals to impose someeffect on the environment either local to or remote from the examplecomputing device 500, such as, physical movement of some object (e.g., amechanical actuator), heating or cooling of a substance, adding achemical substance, and/or the like.

In one implementation, a communication port 510 is connected to anetwork by way of which the computer system 500 may receive network datauseful in executing the methods and systems set out herein as well astransmitting information and network configuration changes determinedthereby. Stated differently, the communication port 510 connects thecomputer system 500 to one or more communication interface devicesconfigured to transmit and/or receive information between the computingsystem 500 and other devices by way of one or more wired or wirelesscommunication networks or connections. Examples of such networks orconnections include, without limitation, Universal Serial Bus (USB),Ethernet, Wi-Fi, Bluetooth®, Near Field Communication (NFC), Long-TermEvolution (LTE), and so on. One or more such communication interfacedevices may be utilized via the communication port 510 to communicateone or more other machines, either directly over a point-to-pointcommunication path, over a wide area network (WAN) (e.g., the Internet),over a local area network (LAN), over a cellular (e.g., third generation(3G) or fourth generation (4G)) network, or over another communicationmeans. Further, the communication port 510 may communicate with anantenna or other link for electromagnetic signal transmission and/orreception.

In an example implementation, particle sample data and software andother modules and services may be embodied by instructions stored on thedata storage devices 504 and/or the memory devices 506 and executed bythe processor 502.

The system set forth in FIG. 8 is but one possible example of a computersystem that may employ or be configured in accordance with aspects ofthe present disclosure. It will be appreciated that other non-transitorytangible computer-readable storage media storing computer-executableinstructions for implementing the presently disclosed technology on acomputing system may be utilized.

In the present disclosure, the methods disclosed may be implemented assets of instructions or software readable by a device. Further, basedupon design preferences, it is understood that the specific order orhierarchy of steps in the methods described herein can be rearrangedwhile remaining within the disclosed subject matter. Any accompanyingmethod claims present elements of the various steps in a sample orderand are not necessarily meant to be limited to the specific order orhierarchy presented.

The described disclosure may be provided as a computer program product,or software, that may include a non-transitory machine-readable mediumhaving stored thereon instructions, which may be used to program acomputer system (or other electronic devices) to perform a processaccording to the present disclosure. A machine-readable medium includesany mechanism for storing information in a form (e.g., software,processing application) readable by a machine (e.g., a computer). Themachine-readable medium may include, but is not limited to, magneticstorage medium, optical storage medium; magneto-optical storage medium,read only memory (ROM); random access memory (RAM); erasableprogrammable memory (e.g., EPROM and EEPROM); flash memory; or othertypes of medium suitable for storing electronic instructions.

It is believed that the present disclosure and many of its attendantadvantages will be understood by the foregoing description, and it willbe apparent that various changes may be made in the form, constructionand arrangement of the components without departing from the disclosedsubject matter or without sacrificing all of its material advantages.The form described is merely explanatory, and it is the intention of thefollowing claims to encompass and include such changes.

The above specification, examples, and data provide a completedescription of the structure and use of example implementations of theinvention. Various modifications and additions can be made to theexemplary implementations discussed without departing from the spiritand scope of the presently disclosed technology. For example, while theimplementations described above refer to particular features, the scopeof this disclosure also includes implementations having differentcombinations of features and implementations that do not include all ofthe described features. Accordingly, the scope of the presentlydisclosed technology is intended to embrace all such alternatives,modifications, and variations together with all equivalents thereof.

What is claimed is:
 1. A system for sampling particles from air, thesystem comprising: a cassette housing formed from a cassette top and acassette bottom; an internal cavity formed within the cassette housing;an inlet opening defined in a proximal end of the cassette top, theinlet opening having an inlet diameter; an internal surface defined inthe top cassette, the internal surface extending along an airflow curvefrom the inlet opening to the internal cavity; a filter having aplurality of pores; and a sampling substrate formed by at least one gridattached to the filter, the sampling substrate disposed in the internalcavity at an internal distance from the inlet opening, the inlet openingand the airflow curve of the internal surface generating an airflow ofthe air to the sampling substrate, the sampling substrate collecting aset of the particles from the air, the inlet diameter, the airflow, andthe internal distance dictating a cutoff diameter of the set ofparticles collected from the air by the sampling substrate.
 2. Thesystem of claim 1, wherein the set of particles are collected directlyon the sampling substrate.
 3. The system of claim 1, wherein the set ofparticles are collected on both the at least one grid and the filterthrough diffusion.
 4. The system of claim 1, wherein the inlet diameteris 2 mm and the internal distance is 6 mm when the cassette housing hasa diameter of 25 mm.
 5. The system of claim 1, wherein the at least onegrid is a film coated transmission electron microscopy grid.
 6. Thesystem of claim 1, wherein the filter is a polycarbonate membranefilter.
 7. The system of claim 1, wherein the filter is supported by asupport screen.
 8. The system of claim 1, wherein the grid is attachedto the filter with tape.
 9. The system of claim 1, wherein a firstportion of the set of particles are collected on the at least one gridand a second portion of the set of particles are collected on thefilter.
 10. The system of claim 9, wherein the second portion includesparticles having a diameter that is greater than that of the firstportion of the set of particles but less than the cutoff diameter. 11.The system of claim 9, wherein the first portion is analyzable directlyby a transmission electron microscope on the grid.
 12. The system ofclaim 9, wherein the second portion is analyzable directly by a scanningelectron microscope on the filter.
 13. A method for sampling particlesfrom air, the method comprising: drawing the air into a cassette housingthrough an inlet opening, the inlet opening having an inlet diameter;generating an airflow of the air to a sampling substrate using the inletopening and an internal surface extending along an airflow curve fromthe inlet opening to the sampling substrate, the sampling substrateformed by attaching at least one grid to a filter and disposed at aninternal distance from the inlet opening; dictating a cutoff diameter ofparticles based on the inlet diameter, the airflow, and the internaldistance; and collecting a set of the particles from the air on thesampling substrate based on the cutoff diameter.
 14. The method of claim13, wherein the set of the particles is collected on the samplingsubstrate through diffusion.
 15. The method of claim 13, wherein one ofthe at least one grid is attached to a center of a surface of the filterproximal to the inlet opening.
 16. The method of claim 13, wherein theair is drawn into the cassette housing using a personal sampling pump.17. The method of claim 13, wherein the cutoff diameter is a mass medianaerodynamic diameter of 3.8 μm.
 18. A method for sampling particles fromair, the method comprising: receiving a first image of a first set ofcollected particles captured using a transmission electron microscope ona grid, the first set of collected particles collected directly on thegrid of a sampling substrate; receiving a second image of a second setof collected particles captured using a scanning electron microscope ona filter, the second set of collected particles collected directly onthe filter of the sampling substrate, the sampling substrate disposed atan internal distance from an inlet opening defined in a cassettehousing, the inlet opening having an inlet diameter, a cutoff diameterof the first set of collected particles and the second set of collectedparticles dictated by the inlet diameter, the internal distance, and anairflow generated using the inlet opening and an internal surfaceextending along an airflow curve from the inlet opening to the samplingsubstrate; determining a particle count of the first set of collectedparticles and the second set of collected particles by analyzing thefirst image and the second image using at least one computing unit;determining one or more particle sizes of the first set of collectedparticles and the second set of collected particles by analyzing thefirst image and the second image using the at least one computing unit;and determining an airborne particle concentration of the air based onthe particle count and the one or more particle sizes.
 19. The method ofclaim 18, wherein the cutoff diameter is a mass median aerodynamicdiameter of 3.8 μm.
 20. The method of claim 18, wherein both the firstset of collected particles are collected directly on the grid the secondset of collected particles are collected directly on the filter throughdiffusion.